Chapter 3: Comparing juvenile chum salmon (Oncorhynchus keta) physiological
3.2 Materials and methods
3.3.2 Biophysical variables
In both habitats, 2011 was warmer than 2010 and the EGOA was warmer than the strait habitat (strait- 2010: 9.46°C +/- 0.19, 2011: 9.90°C+/-0.24, EGOA- 2010: 11.51°C+/- 0.71, 2011: 12.11°C+/- 0.78). The EGOA was more saline than the strait habitat, but there were no differences in salinity between years. There was a significantly higher concentration of chl-a in 2010 compared to 2011 (GLS, p = 0.004) and an interaction between years and habitat (GLS, p = 0.014; Table 3.4). There were no significant differences in MLD between years or habitat.
For zooplankton, there was not a significant interaction effect in the model and so the interaction terms were dropped from the full model (Eq. 3.2). Zooplankton density was significantly higher in the strait habitat than in the EGOA off Icy Point (ANOVA, p < 0.005), and although 2011 had a higher density of zooplankton, there was no significant difference between years (Fig. 3.3).
3.3.3 Physiological status variables
The length-weight relationship for juvenile chum salmon followed the equation
In W = -11.95 + 3.10 * In L 3.4
with the residuals representing the weight-at-length residuals specific to each fish sampled. Samples were pooled from both years (N = 479 in 2010; N = 237 in 2011) and habitats (N = 510 in strait; N = 206 in EGOA).
To put in context the spatial and temporal dataset used in this observational study, the
untransformed length measurements of juvenile chum salmon collected from both habitats for all available sampling months were compared. The average length of fish in July in the strait habitat was similar to the average length of fish in July in the EGOA habitat (strait = 124.24 mm, EGOA = 123.03 mm). The observation that juvenile chum salmon were similar in length in both habitats
in July led us to believe that the biophysical parameters associated with these habitats could also be compared in July to assess differences in habitat characteristics.
Linear mixed effect models compared physiological status response variables between habitats, years and stocks (except for energy density) and summaries of model results are found in Table B-1. There were interactions in the linear mixed effect model for the response variable length and therefore we analyzed the habitats separately. In both habitats, there was a significant difference in the length of juvenile chum salmon between stocks (strait: p < 0.001, EGOA: p = 0.05, wild shorter than hatchery), but no significant difference between years (Fig. 3.4). There were no significant interaction effects in the full models for both habitats with the best models being the reduced model with no interaction terms (LME, Eq. 3.3; Table B-2).
For weight-at-length residuals, there was a significant difference between years (higher weight-at-length residuals in 2010 than 201 1, p < 0 .001) habitats (higher weight-at-length residuals in EGOA than Icy Strait, p < 0.001) and stocks (higher weight-at-length residuals of wild than hatchery stocks, p = 0.001; Fig. 3.5). There were no significant interaction effects in the full model (LME, Eq. 3.3; Table B-2).
For energy density, in both years, there was a significant difference between habitats with fish in the EGOA having higher energy than in the strait habitat (2010: p = 0.004; 2011: p < 0.001; Fig. 3.6). By habitat, there was a significant difference in energy density between years in the EGOA (2011 higher than 2010, p = 0.009) but not in the strait habitat. There was a
significant interaction between habitat and year (LME, Eq. 3.3, p = 0.037; Table B-2). The variability among and between stations was similar in the full model with interaction terms.
3.4 Discussion
For chum salmon production response variables, juveniles that entered the ocean in 2010 compared to 2011 had higher adult returns and survival to SEAK when lagged three ocean years. Commercial harvests of chum salmon to SEAK were 10.2 M fish in 2013 and 6.5 M fish in 2014 (Conrad and Gray, 2014). Furthermore, actual marine survival of age-4 chum salmon to the DIPAC hatchery was fourfold higher for fish entering the ocean in 2010 (2009 brood year, 3.25%) compared to 2011 (2008 brood year, 0.77%). In 2013, hatchery chum salmon (those that would have outmigrated in 2010) comprised 80.5% of the commercial common property harvest
harvest, while in 2014 (those that would have outmigrated in 2011) comprised 85.3% of the total, indicating that there was higher survival for wild chum salmon from the ocean-entry year of 2010 compared to 2011 (Vercessi, 2013, 2015).
In this comparison study, we found that ocean-environment conditions associated with a strong AL: lower NPI, higher MEI (warm spring SST), higher freshwater discharge and stronger coastal downwelling coincided with higher weight-at-length residuals in the out-migrating juveniles in summer as well as higher commercial harvest and hatchery survival lagged 3 years for returning adult chums in SEAK. The link between weight-at-length residuals and adult returns was also found in another GOA study finding that hatchery juvenile pink salmon that were heavier at a given length had higher survival (Miller et al., 2012). The result that commercial harvest and hatchery survival of age-4 fish were higher (50% and 200% higher, respectively) for juveniles entering the ocean in 2010 compared to 2011 supports the concept of using physiological status data for juvenile chum salmon as a potential predictive salmon management tool to forecast year class strength in SEAK.
Response variables measured could be influenced by the migration timing of stocks of juvenile salmon in northern SEAK. In Icy Strait there was a higher percentage of hatchery stocks in 2010 compared to 2011 (58% vs. 51%). The higher percentage of hatchery chum salmon in Icy Strait in 2010 compared to 2011 is consistent with the higher marine survivals reported for DIPAC age 4 fish released in 2010 (3.25%) compared to 2011 (0.77%). Conversely, in the EGOA, the proportion of hatchery juvenile chum was lower in 2010 compared to 2011 (60% vs. 70%). The difference in stock composition (hatchery/wild) between habitats in both years can be also explained by the trend for some hatchery stocks of chum salmon (i.e. DIPAC) to have peak migrations through Icy Strait in June (Orsi et al., 2005). At the time of the survey in Icy Strait (late July) the high peaks of hatchery-marked juvenile chum salmon had already migrated through Icy Strait to the EGOA.
The result that wild stocks were heavier at a given length and had shorter overall lengths compared to hatchery stocks could be due to the difference in foraging strategies between
hatchery and wild fish (Sturdevant et al., 2010) or that there was a difference in migration timing of wild and hatchery fish stocks. The only indicator available to assess the difference in adult
returns between hatchery and wild chum salmon was the composition of the commercial harvest in SEAK. The commercial harvest of chum salmon in 2013 (2010 ocean-entry year) had the lowest percentage of hatchery fish in the past decade (2004-2014) indicating high returns of wild chum to SEAK. High hatchery survival and a commercial catch comprising a higher percentage of wild chum salmon suggests that 2010 had favorable growing conditions for juvenile chum salmon.
Conversely, in 2011, the measured ocean environmental characteristics depicted a weak AL; higher NPI, low MEI, low freshwater discharge and relaxed downwelling coinciding with higher energy densities in the EGOA in July in 2011. Energy density measured in the summer growing season was difficult to interpret as an indicator of physiological status and subsequent production of salmon. Water temperature could also influence the allocation of energy, with fish having higher energy densities when sea temperatures are cooler (Heintz, 2009). The colder spring sea temperatures in 2011 could have influenced the growing conditions for juvenile chum salmon previous to collection in July. The contradiction between energy density and the other response variables suggests that measurements of physiological status were affected by different
mechanisms specific to the fish life history at the time and location of collection.
When comparing habitats, the low weight-at-length residuals in the strait habitat compared to the EGOA may be the result of juvenile chum salmon not allocating energy to lipid stores, but to avoiding predation or maintaining basic body functions corresponding to being at an earlier developmental stage in the strait habitat. These results contradict those of a similar previous study in SEAK where condition (measured as condition factor K) was found to be higher in stocks in the strait habitat compared to coastal habitat (Orsi et al., 2001). Our result that fish in the EGOA were heavier for their length could indicate that the coastal habitat intersects the right time in a juvenile chum salmon’s life for energy to be allocated to lipid storage rather than somatic growth. This physiological transition coincides with the early ocean life history of chum salmon, which grow rapidly in spring and early summer in strait and coastal habitats, then later occupy the EGOA in the late summer and fall as they need to store energy and overwinter.
observed in 2010 supports a previous study examining chum salmon in SEAK that found spring freshwater discharge was a promising positive correlate of survival and harvest (Orsi and Fergusson, 2009). Higher downwelling values, as seen in 2010, support a companion study finding a positive correlation between the abundance of juvenile chum salmon in SEAK and downwelling intensity (Chapter 2).
Low freshwater influx, as observed in 2011, could have decreased the levels of bioavailable iron, primarily sourced from freshwater rivers in SEAK, and subsequently prevented offshore transport to stimulate primary production in outer shelf waters (Martin and Gordon, 1988; Wu et al., 2009). Waite and Mueter (2013) found that positive spring chl-a concentration anomalies were associated with lower spring SST and increased upwelling (relaxed downwelling) in the EGOA, characteristics of a weak AL. However, in spite of the cool SST and relaxed
downwelling conditions in the spring of 2011, satellite-derived chl-a anomalies were much lower in the spring and particularly in the fall of 2011 compared to 2010 (Waite and Mueter, 2013).
Similarly, in situ chl-a values were significantly lower in the EGOA in July 2011 than in July 2010, despite higher chl-a values in the strait habitat in 2011. The shallow MLDs, low chl-a concentrations, and low freshwater discharge rates as seen in 2011 in the EGOA, could have negatively influenced the timing of stratification and amount of primary production, creating a match-mismatch situation for prey resources and juvenile salmon in the EGOA for this year.
Although the relationship was not significant, there were higher densities of zooplankton in 2011compared to 2010 which could explain the low primary production in 2011 in the EGOA resulting from grazing pressure by zooplankters effectively limiting the overall production of primary producers (Strom, 2001). Because the production of zooplankton biomass lags primary production by 1-2 months (Cooney, 1988), the sampling design for this study might not capture the true habitat characteristics for each sampling year. These observations suggest that the mechanisms driving productivity can vary over relatively small spatial and temporal scales.
In conclusion, differences in juvenile chum salmon physiological status in 2010 and 2011 coincided with positive and negative anomalies of the coupled ocean-atmosphere system as well as chum salmon harvest and survival lagged three ocean years. These differences suggest that the use of previous winter environmental conditions at both the basin and regional scale and juvenile
chum salmon physiological status have potential to be used as predictive tools for forecasting salmon year class strength in SEAK.
3.5 Figures
138
-137
-136
-135
-134
Longitude
Figure 3.1 Map of sampling stations in the Eastern Gulf of Alaska (EGOA) and Icy Strait. EGOA stations are represented by red circles (2010) and orange triangles (2011). Icy Strait stations are represented by blue open squares (2010 and 2011).
Multivariate ENSO Index
2 1.5 V I— 3 1r.
ro a . 0 .5 <v O 0 73 <U N -0.5 ■o re -1 •u c re -1 .5 co -2 -2.5 Monthly M easurementFigure 3.2 Comparison of the standardized departures from the mean monthly measurements of the multivariate ENSO index for 2010 (blue) and 2011 (green) (Wolter, 2013, data source: www.esrl.noaa.gov/psd/enso/mei/).
3
Figure 3.3 A boxplot depicting the differences in zooplankton density (ml/m ) between year and habitat. The boxplot shows median, interquartile range and individuals outside of the
Figure 3.4 Boxplots of ln(length) of juvenile chum salmon in both habitats a) Icy Strait and b) EGOA. The boxplots depict the median and upper and lower quartiles and individuals outside of the interquartile range of the raw data not accounting for a station effect for year and stock. Blue boxes indicate wild stocks and clear boxes indicate hatchery stocks. The red line represents the modeled mean after accounting for a station effect. Outliers were removed from the data to estimate the means.
Figure 3.5 A boxplot of juvenile chum salmon weight-at-length residuals. The boxplot depicts the median and upper and lower quartiles and individuals outside of the interquartile range of the raw data not accounting for a station effect. The red line represents the modeled mean after accounting for a station effect. Blue boxes indicate wild stocks and clear boxes indicate hatchery stocks. Outliers were removed from the data to estimate the means.
Figure 3.6 A boxplot of juvenile chum energy density (j/g ww). The boxplot depicts the median and upper and lower quartiles individuals outside of the interquartile range of the raw data not accounting for a station effect. The red line represents the modeled mean after accounting for a station effect. Outliers were removed from the data to estimate the means.
Table 3.1 Differences in trawl sampling effort in the Eastern Gulf of Alaska for July of 2010 and 2011.
3.6 Tables
Trawl Sampling Effort 2010 2011
Trawl date 7/4-7/20 7/3-7/17
Grid Direction N->S S->N
Number of hauls 27 27
Trawl gear Nordic Cantrawl
Trawl Dimensions
(m, WxH) 20x20 40x30
Head Rope Spread (m ) 400 1200
Trawl Speed (m/s) 2.8 3.4
Trawl time (min) 30 30
Distance (m) 302,400 367,200
Table 3.2 Eastern Gulf of Alaska oceanographic characteristics measurements obtained from stations in 2010 and 2011.
Year Characteristic # of stations sampled
2010 Temperature 27 2011 Temperature 20 EGOA 2010 Salinity 27 2011 Salinity 20 2010 Chlorophyll 27 2011 Chlorophyll 14 2010 Temperature 4 2011 Temperature 4 Strait 2010 Salinity 4 2011 Salinity 4 2010/2011 Chlorophyll 4
Table 3.3 Possible ecosystem indices as drivers for juvenile chum salmon physiological status compared between 2010 and 2011. The Nov-Mar time period is the winter prior to the ocean year.
Environmental Data
Variable Scale Time Period 2010 2011
MEI Basin Nov-Mar 1.23 -1.60
NPI Basin Nov-Mar 1006.49 1011.13
FW Discharge (ft3/sec) Regional Mar-May 4,531.76 11,804.26
Table 3.4 Generalized least-squares model generated means for chlorophyll concentration (pg/L) and mixed layer depth (MLD). Data is from stations sampled in Icy Strait and the Eastern Gulf of Alaska in July of 2010 and 2011.
Habitat Characteristic Year # of stations Model Mean
Chlorophyll 2010 4 2.10 Chlorophyll 2011 4 4.11 Strait MLD 2010 4 6.21 MLD 2011 4 6.16 Chlorophyll 2010 27 2.52 EGOA Chlorophyll 2011 14 1.69 MLD 2010 27 9.20 MLD 2011 20 6.39
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